Fundamental primitives of XSLT programming


A friend planning an introductory course on programming for linguists recently asked me what I thought such linguist-programmers absolutely needed to learn in their first semester of programming. I thought back to a really helpful “Introduction to Programming” course taught in the 1980s at the Princeton University Computer Center by Howard Strauss, then the head of User Services. As I remember it, it consisted essentially of the introduction of three flow-chart patterns (for a sequence of steps, for a conditional, and for a while-loop), with instructions on how to use them that went something like this:

  1. Start with a single box whose text describes the functionality to be implemented.
  2. If every box in the diagram is trivial to implement in your programming language, stop: you’re done. Implement the program using the obvious translation of sequences, loops, and conditionals into your language.
  3. Otherwise choose some single box in the diagram whose functionality is non-trivial (will require more than a few lines of code) and replace it with a pattern: either break it down into a sequence of steps, or make it into a while-condition-do-action loop, or make it into an if-then-else choice.
  4. Return to step 2.

I recommended this idea to my friend, since when I started to learn to program I found these three patterns extremely helpful. As I thought about it further, it occurred to me that the three patterns in question correspond 1:1 to (a) the three constructors used in regular languages, and (b) the three patterns proposed in the 1970s by Michael A. Jackson. The diagrams I learned from Howard Strauss were not the same as Jackson’s diagrams graphically, but the semantics were essentially the same. I expect that a good argument can be made that together with function calls and recursion, those three patterns are the atomic patterns of software design for all conventional (i.e. sequential imperative) languages.

I think the patterns provide a useful starting point for a novice programmer: if you can see how to express an idea using those three patterns, it’s hard not to see how to capture it in a program in Pascal, or C, or Python, or whatever language you’re using. Jackson is quite good on deriving the structure of the program from the structures of the input and output in a systematic way.

The languages I most often teach, however, are XSLT and XQuery; they do not fall into the class of conventional sequential imperative languages, and the three patterns I learned from Howard Strauss (and which Howard Strauss may or may not have learned from Michael A. Jackson) do not help me structure a program in either language.

Is there a similarly small set of simple fundamental patterns that can be used to describe how to build up an XSLT transformation, or an XQuery program?

What are they?

Do they have a plausible graphical representation one could use for sketching out a stepwise refinement of a design?